1. Field of the Invention
This invention relates to an injection control method for a die cast machine.
2. Description of Related Art
It is known that the quality of die cast products may be greatly influenced by the injection speed and the injection pressure when the molten metal is filled into a mold cavity.
In a conventional die cast machine, a two-stage type drive cylinder is employed to obtain satisfactory pressure before the molten metal solidifies.
In a typical die cast machine 90, as shown in FIG. 10, a molten metal 92 is fed into an injection sleeve 93 and is then injected into a mold cavity 91 by a plunger 94. A hydraulic fluid on the rear side of an injection cylinder 95 is pressurized by an oil-pressure intensifier 96 having a large diameter as the injection process is completed, so that the molten metal 92 in the mold cavity 91 is further pressurized.
An injection speed progression of the die cast machine 90 is shown in FIG. 11(B). The advance speed of the injection cylinder 95 is slow at first and increases rapidly after passing time t.sub.1. Incidentally, the speed of the injection cylinder 95 decreases according to the pressure increase of the molten metal 92. At time t.sub.2, the oil-pressure intensifier 96 is activated to further advance the injection cylinder 95 until time t.sub.3 when the advancement thereof is terminated. The termination point of the advancement of the injection cylinder 95 is defined as a stroke-end position Dse.
In FIG. 11(A), an injection pressure progression of the die cast machine 90 is shown. A pressure curve L1 increases rapidly due to the advancement of the oil-pressure intensifier 96 after the time t.sub.2 when the injection process is completed. Intensifier 96 ceases operation at t.sub.4.
The link control of cylinders 95 and 96 in the die cast machine 90 is generally conducted by a sequential-valve control as shown in FIG. 12 or a limit-switch control as shown in FIG. 13.
FIG. 12 illustrates an oil-pressure circuit 114 having a check valve 111 and a speed-control valve 112 coupled between an accumulator 113 and the injection cylinder 95. FIG. 12 further illustrates an oil-pressure circuit 117 having a pilot-type intensify valve 116 and a sequential valve 115 coupled between the oil-pressure intensifier 96 and the accumulator 113. The sequential valve 115 is set to open the intensify valve 116 when the pressure in the oil-pressure circuit 114 of the injection cylinder 95 exceeds a predetermined boost-start point. As a result, the injection cylinder 95 is advanced as controlled by the speed-control valve 112 to inject the molten metal into the cavity. The sequential valve 115 is actuated after the injection process is completed to open the intensify valve 116 and the oil-pressure intensifier 96 is advanced to further pressurize (i.e. "boost") the molten metal injected in the mold cavity.
In FIG. 13, the injection cylinder 95 is shown as having a check valve 121 which penetrates the oil-pressure intensifier cylinder 96 along the center axis thereof and an accumulator 123 which includes a change valve 122 at the flow passage to the injection cylinder 95. The oil-pressure intensifier 96 is also shown as having an accumulator 126 provided with the change valve 125 controlled by a magnetic valve 124 which is connected to a limit switch 127. The limit switch 127 is disposed where it may contact projection 129 of rod 128 just before the injection process is completed. Accordingly, the injection cylinder 95 is further advanced as controlled by the change valve 122 and the oil-pressure intensifier 96 may be advanced as controlled by the limit switch 127 just before the injection process of the injection cylinder 95 is completed.
However, in the above die cast machine 90, to achieve the preferable pressure curve L1 shown in FIG. 11(A), it is necessary to perform link control of cylinders 95 and 96 with accuracy especially to set a time for the advancement of the oil-pressure intensifier 96.
For example, if the advancement of the oil-pressure intensifier 96 occurs too soon before completion of the injection of the molten metal by the injection cylinder 95, the desired boosting may not occur as indicated by pressure curve L2 as shown in FIG. 11(A). Also, if the advancement of the oil-pressure intensifier 96 is late, the pressure boost in the cavity 91 starts after the advancement of the injection cylinder 95 is completed, and thus the molten metal 92 in the cavity 91 has begun to cool and the desirable effect may not be obtained as shown by pressure curve L3 in FIG. 11(A). In addition, it is difficult to change the time of the advancement of the oil-pressure intensifier 96 effectively by the conventional link control method.
Under the sequential-valve control method of in FIG. 12, if a boost start pressure of the sequential valve 115 is set low, the sequential valve 115 may be incorrectly activated before the injection process by the injection cylinder 95 is completed. This may occur because a pressure peak appears when the injection speed changes from low to high, or an unintentional increase of injection pressure occurs due to resistance between the injection sleeve and the plunger. Alternatively, if the boost start pressure is set high, a time lag occurs because the advancement of the oil-pressure intensifier 96 may not began until after the completion of the injection process. Furthermore, since the injection pressure should be altered to correspond to different types of molding dies, it is necessary to correct the setting pressure of sequential valve 115 every time a different molding die is used.
Under the limit-switch control method shown of FIG. 13, the stop position of the plunger differs with regard to different kinds of molding dies, thus requiring that the setting position of the limit switch 127 be adjusted for each corresponding die. Also, even if a molding die is not changed, if the amount of molten metal 92 supplied to the injection sleeve 93 as shown in FIG. 10 varies, the thickness of the metal 92 ("biscuit") remaining in the injection sleeve 93 after the completion of the injection process may vary. Accordingly, the position detected by the limit switch 127 does not always represent a position just before the completion of the injection process by the injection cylinder 95, thus the boost start position may not be reliable. Also, when the divergence of the change valve 125 of the oil-pressure intensifier 96 is increased, the advancement of the oil-pressure intensifier 96 is increased so that the advance stroke thereof may be longer than a desired stroke length or may reach the stroke end position Dse too quickly, thus the boosting may not be correctly achieved.
During conventional injection by an injection plunger in an injection cylinder, the injection speed is variable in one casting cycle to attain a desirable injection. For example, with reference to FIG. 14(B), an injection speed VL between an injection start time T.sub.0 and time T.sub.1 is slow to prevent a wave of the molten metal in the injection sleeve. An injection speed VH between the time T.sub.1 and time T.sub.2 is more rapid to complete the injection of the molten metal into the cavity before solidification. As the injection process is competed as shown in FIG. 14(A), the injection pressure is increased by a pressure P.sub.2, so that the molten metal in the cavity is quickly boosted and solidified. Also, the injection plunger is further advanced until time T.sub.3.
When reviewing the injection pressure progression in the above injection mode, the pressure in a low-speed injection zone is low (PL) and in a high-speed injection zone is high (PH) as shown in FIG. 14(A). Accordingly, if the injection speed remains high until the injection completion time T.sub.2, an impulsive surge pressure Ps may be caused by the reaction force of the molten metal completely filling up in the cavity, so that the molten metal may tend to emit from the parting line of the die and undesirable forming such as a flash may appear.
It is therefore recommended to slow down the injection speed around a certain time T.sub.4 which is near to the end of the high-speed injection zone and to reach the low speed Vd at the completion of the injection process as shown by a dotted line in FIG. 14(B). Such a slow-down control may eliminate the surge pressure Ps at the completion of the injection process, so that the smooth boosting indicated by the dotted line in FIG. 14(A) may occur.
It is required for the described slow-down control to slow down the injection speed until the completion of the injection process at time T.sub.2. However, it is difficult to accurately monitor the injection process in the cavity. It may be possible to detect the injection completion time T.sub.2 based upon the steep rising of the injection pressure shown in FIG. 14(A), but it is difficult to detect a prior time, that is, the slowing-down start time T.sub.4 before the injection completion time T.sub.2.
Accordingly, since the injection condition in the cavity seems to correspond to the advanced position of the injection plunger or the injection cylinder, the following method is introduced, in which the slowing-down of injection speed is conducted based on detection of the mechanical position of the injection plunger or injection cylinder.
In FIG. 15, it is known that an injection cylinder 60 has a piston 61 therein. The piston 61 is connected to an injection plunger 74 at its rod 61A by a coupling 77. Hydraulic piping is provided on the rear side of the piston 61 coupled into the injection cylinder 60. The piping 63 constantly contains a hydraulic fluid from an oil source 64 or an accumulator 65. Hydraulic piping 66 is connected on the front side of the piston 61 of the injection cylinder 60. The piping 66 is further connected to an oil tank 67 through a change valve 70, or to hydraulic piping 68 which is connected to the piping 63.
The change valve 70 may couple the piping 66 with the oil tank 67 as a solenoid 71 is excited. Low-speed injection may be performed by reducing the flow rate in a control valve 72. If solenoid 73 is excited, the piping 66 and 68 are coupled together with piping 63 to thereby advance the piston 61 quickly to perform high-speed injection. When both solenoids 71 and 73 are not excited, the piping 66 is coupled to the tank 67 through the restrictor 76 and the following flow rate control valve 72 so that the discharge of the pressured oil contained on the front side of the injection cylinder 60 is restricted to thereby slow down from high-speed injection.
To execute the above shifting of the change valve 70, a projection 77a is provided at one end of a coupling 77 of the piston rod 61A of the injection cylinder 60. Also, two corresponding limit switches 78 and 79 are provided. The limit switch 78 is positioned to contact the projection 77a at the time T.sub.1 to switch the change valve 70 to the high-speed injection mode. The limit switch 79 is positioned to contact the projection 77a at the time T.sub.4 to switch the change valve 70 to the low-speed injection mode.
Under such an arrangement of the injection cylinder 60, the piston 61 is first retracted to its retract limit position and the change valve 70 is shifted to the low-speed injection mode. The change valve 70 is automatically changed over by the limit switch 78 in accordance with the advancement of the piston 61 to thereby start the high-speed injection mode. The following limit switch 79 switches the change valve 70 to conduct the slowing-down mode.
In any conventional die cast machine, the supply amount of the molten metal varies because the molten metal is poured into the injection sleeve by a ladle at the beginning of every casting cycle. Such variation influences the advance volume or injection process completion position of the injection plunger or injection cylinder for every casting cycle. However, using the above mechanical detection system, it is difficult to adjust the positions both of the projection and the limit switch 79 to correspond to the injection completion position that may vary with every casting cycle. Hence, the precise slowing-down start timing may not be obtained.
As discussed above, it is desirable to adjust for the fluctuation of the molten metal supply amount at every casting cycle when performing the link control of the two-stage type injection cylinder which consists of the injection cylinder and the oil-pressure intensifier, or when performing the slowing-down control of the injection cylinder.